1. Field of the Invention
The present invention relates to apparatus and methods of acoustic flow measurement using acoustic pulses to derive fluid flow rate.
2. Background Information
The prior art teaches ultrasonic time-of flight flow meters which rely on acoustic waves propagating in the flowing fluid. These devices usually use piezoelectric transducers to convert electrical drive signals to the propagated acoustic energy and to convert the received acoustic energy to electrical signals for processing to derive the fluid flow rate.
Piezoelectric transducers are typically made of ceramic materials such as lead zirconate titanate which, while relatively economical and efficient, have mechanical properties that are significantly different from those of the flowing fluid with which they are interfacing. These differences result in poor acoustic coupling or impedance mismatching between the transducer and fluid, which in turn necessitates the use of higher gain signal amplifiers and more elaborate noise filtering and signal processing which add cost to the meter. The mismatch becomes extreme when ceramic piezoelectric transducers are used to measure flow of gases, such as steam or stack gases. Furthermore, the elevated temperatures sometimes encountered in measuring flow of gases can pose an additional incompatibility.
Therefore, there is a need for cost effective apparatus and methods for minimizing transducer-to-fluid mismatching and for increasing the transducer operating temperature in acoustic time of flight flow meters.